This invention relates to an optical scanning apparatus for the planar scanning of a given test specimen with a beam of light such as an ultraviolet ray which is electromagnetically uncontrollable.
Among the microscopes which are capable of magnifying a given test specimen with sufficiently high resolvability to permit visual observation, there are included microscopes of the type which make use of an electron beam. In the scanning electron microscope which is typical of these microscopes, the observation of a given test specimen is accomplished by concentrating the primary electron beam into a small focal point, causing the focused electron beam to scan the surface of the test specimen regularly and repeatedly, collecting the secondary low-energy electrons produced by the interreaction between the incident electron beam and the test specimen, modulating the collected secondary electrons as an electric signal in terms of the brilliance of the cathode-ray tube (hereinafter referred to as "CRT") and, thereby, enabling the test specimen to be viewed in a magnified image. The primary electron beam which is used in this type of microscope is first deflected by the Lorenz force in the electromagnetic field which is generated by application of an exciting current to a deflecting coil and then used to scan the surface of a given test specimen. Having no need for any mechano-optical driving device, this method of scanning can be easily operated with high accuracy at a high speed. Although the scanning electron microscope operated by this principle enjoys high resolving power, microscopes having still higher degrees of resolving power have been desired in various technical fields. To meet this desire, there has been proposed a new type of microscope which uses an ultraviolet beam as the source of irradiation in the place of the electron beam. The reason for this switch in the source of irradiation is that the resolving power such as is obtained in the microscope depends on the wavelength of the source of irradiation. In this respect, it is reasonable to conclude that light in the ultraviolet zone, which has smaller wavelength, is effective in increasing the resolving power. Besides, ultraviolet beam is characterized by possessing higher energy and suffering from a lower attenuation factor.
In this type of microscope, when the test specimen is irradiated by electrons as the source of irradiation, secondary electrons consequently emitted from the surface of the test specimen are collected. When the ultraviolet beam is used as the source of irradiation, there ensues generation of exoelectrons from the surface of the test specimen. For the two types of radiant beams, although the treatments to which they are subjected are virtually identical, the methods by which they are utilized for scanning the surface of the test specimen are different. The electrons can be electromagnetically controlled such as with a deflecting coil, whereas the light of the ultraviolet zone used as the source of irradiation cannot be electromagnetically controlled.
For the planar scanning of the test specimen to be effectively carried out by using, as the source of irradiation, the beam of an electromagnetically uncontrollable light such as the ultraviolet light, it has been customary to adopt either a method which causes the test specimen to be mechanically moved relative to the path of the light beam or a method which optically deflects the incident ultraviolet beam. Generally, the former method which requires the test specimen to be moved by a suitable driving device and permits the incident ultraviolet beam to be kept stationarily is adopted more often than the latter method. In the former method of scanning, however, the test specimen itself must be moved and, therefore, must be provided with an external load function. This method, accordingly, has the disadvantage that the driving device inevitably incorporated is too massive to permit speedy scanning of the test specimen.
In order for the latter method to be advantageously adopted, there has been conceived an idea of utilizing a reflecting mirror which is disposed across the optical axis of the incident ultraviolet beam and adapted to be oscillated by some suitable means. In this connection, optical scanning devices applicable to the aforementioned scanning microscopes have been proposed. None of these devices, however, have structures which are suitable for practical uses. Since, in the microscope, the image of the test specimen to be observed therethrough is reproduced by the CRT, the reflecting mirror is required to be linearly oscillated so as to permit ready synchronization with the CRT scanning and provides high-speed linear scanning of the test specimen. In actuality, however, one of these optical scanning devices produces an elliptic locus of scanning on the surface of the test specimen, which is not suitable for the purpose of synchronization with the CRT (Japanese Patent Publication No. 29022/1964) and another of them utilizes a multiplicity of toothed wheels as means for the oscillation of the reflecting mirror at the cost of operating speed and produces a locus of scanning in the shape of a sine curve which similarly is undesirable for the purpose of synchronization with the CRT (Japanese Patent Public Disclosure No. 17244/1974). There has also been proposed a concept of adopting an optical system incorporating two or more reflecting mirrors. None of the devices so far proposed, however, has fulfilled the ideal of enjoying simplicity of structure, exhibiting linearity of scanning speed and providing accurate, high-speed scanning.
The scanning devices developed to date have a serious disadvantage that no free variation can be obtained either in the motion of the reflecting mirror or in the area of scanning zone of the linear density of scanning.
The object of this invention is to provide an optical scanning apparatus which utilizes for the planar scanning of a test specimen the beam of an electromagnetically uncontrollable light, specifically the ultraviolet light, and which provides accurate, high-speed scanning with ample speed linearity and permits free variation in the area of scanning zone and the linear density of scanning.